This invention relates to electric power utility networks including generating systems, transmission systems, and distribution systems serving loads. The power flowing on these networks is primarily in the form of alternating current and as such is familiar to those skilled in the art.
To remain competitive, electrical utility companies continually strive to improve system operation and reliability while reducing costs. To meet these challenges, the utility companies are developing techniques for increasing the life of installed equipment, as well as, diagnosing and monitoring their utility networks. Developing these techniques is becoming increasingly important as the size and demands made on the utility power grid continue to increase.
A utility power grid is generally considered to include both transmission line and distribution line networks for carrying voltages greater than and less than about 25 kV, respectively.
Referring to FIG. 1, a portion of a utility power network is shown to include a transmission network 10 having generators 12, substations 14, and switching stations 16, all of which are interconnected via transmission lines 18. Transmission lines 18, in general, carry voltages in excess of 25 kilovolts (kV). With reference to FIG. 1, the voltage on a particular transmission line is approximately proportional to the thickness of the associated line in the figure. The actual transmission system voltages are indicated in the accompanying key located at the lower right.
Referring to FIG. 2, an exploded portion 10a of the utility power network of FIG. 1 includes distribution lines 20 coupled to a transmission line 18 through step-down transformers 22. Each distribution line carries power to loads 24 at voltage levels less than those levels associated with transmission lines (e.g., 25 kV or less).
The utility power grid is susceptible to faults or contingencies which are a critical problem for the utility industry. In particular, when a fault occurs on the transmission grid, momentary voltage depressions are experienced, which may be problematic to loads connected to the grid.
Large industrial plants with loads above a few megawatts are typically serviced at medium voltage, 4,106V and above, and may have more than one source substation, or have more than one feeder line between the utility and their main transformer(s). While this configuration greatly improves the overall continuity of the power supply, it exposes the plant loads to short duration voltage sags caused by faults or weather related events on the parallel feeders or substations, or on the transmission system. These sags generally fall in the range of 0.2 P.U. to 0.8 P.U. of nominal voltage, and  less than 1 second duration, although there are considerable differences from one location to another.
Most industrial sag events last less then 20 cycles or so. Yet for modern manufacturing facilities, this is more than enough to cause interrupts, especially in automated manufacturing operations, to the point where the user feels a clear cost for these power quality (PQ) events. But with load levels in the megawatt, or 10""s of megawatts range, the cost of available devices to address the sag issue had formerly been far too high to contemplate.
To better understand the dynamics of a fault on a utility power system, the sequence of events on the system due to a 3-phase fault on the transmission system will now be described. For example, referring again to FIG. 1, assume the fault occurs on a portion of the transmission network remote from a segment 70. Segment 70 lies between a substation 14a and a switching station 16 of transmission line network 10. Referring to FIG. 3, the voltage profile as a function of time at substation 14a due to the fault is shown. In this particular case, the voltage drops from a nominal 115 kV to about 90 kV. It is important to appreciate that if the fault were to occur more closely to segment 70 or on the segment itself, the drop in voltage is generally much more severe, and the voltage on the line can approach zero.
In general, such a fault appears as an extremely large load materializing instantly on the transmission system. Further details as to the events which typically occur on the transmission system in response to the appearance of this very large load, are described in U.S. application Ser. No. 09/449,435.
As discussed above, faults occurring on the utility power network have dramatic effects on the loads connected to the distribution network. Indeed, momentary voltage sags at a factory or manufacturing facility can cause production losses, scrap product, missed schedules, overtime and added maintenance, all of which add significant cost. For example, a single power failure at a semiconductor processing facility can result in the scrapping of $250,000 in semiconductor integrated circuits. Moreover, regardless of how well electric utility companies serve such factories, such events are inevitable.
Various equipment and device solutions have been developed to address these momentary voltage sags. In general, such equipment and devices mitigate the effects of these sags by injecting real and/or reactive power into the system.
Two such devices used to address grid instability problems and associated sags are the superconducting magnetic energy storage (SMES) and the PQIVR system, both products of American Superconductor Corporation, Westborough, Massachusetts. The PQIVR system focuses on maintaining power quality for a particular load and integrates energy storage and power electronics to boost utility sag events by 10-50%, at any load level to keep the load operational. A PQIVR can include a superconducting magnet which stores energy used to bridge voltage sags. When a sag is detected, the PQIVR immediately rebuilds the voltage so that the load sees only smooth, uninterrupted power. In some embodiments, the PQIVR can bridge multiple, rapid-fire events and, following any magnitude of carryover, recharge rapidly.
A SMES device is similar to the PQIVR in that it stores electrical energy in a superconducting magnet. However, unlike the PQIVR, the SMES focuses on stabilizing the entire utility power grid instead of concentrating on one industrial customer. In particular, the SMES provides power to the distribution network to stabilize the utility power network in response to a detected fault after the load is isolated from the grid. Because the SMES, like a battery, is a DC device, a power conditioning system is generally required in order to interface it to an AC utility grid. Thus, the power conditioning system generally includes DC/AC converters as well as other filtering and control circuitry.
The invention features an approach for providing voltage protection to a load connected to a distribution network of a utility power system or network by boosting the voltage on a distribution line, during a momentary voltage sag caused by a fault or other contingency. By xe2x80x9cutility power system or networkxe2x80x9d, it is meant those systems or networks having at least one distribution line network coupled to a higher voltage transmission line network designed to carry a nominal voltage under normal operating conditions. The distribution line network generally includes at least one distribution line having a load and carries voltages at levels lower than those on the transmission network.
One general aspect of the invention relates to a method of providing voltage protection from a voltage recovery system to a load connected to a distribution network of a utility power network. The method includes selecting a voltage protection characteristic required by the load; and determining, on the basis of electrical characteristics of the voltage recovery system and the distribution network, whether the voltage recovery system is capable of providing the required voltage protection characteristic.
Embodiments of this aspect of the invention may include one or more of the following features.
Determining whether the voltage recovery system is capable of providing the required voltage protection characteristic includes the following steps. A fault current capability characteristic of the distribution network is determined. This characteristic is commonly referred to as the available fault current of the distribution line or the xe2x80x9cfault MVAxe2x80x9d of the distribution line. The maximum voltage improvement characteristic that the voltage recovery system can provide is also determined. The voltage protection characteristic (e.g., required by the customer) is compared with the maximum voltage improvement characteristic to determine whether the voltage recovery system is capable of providing the required voltage protection characteristic. If the voltage protection characteristic is greater than the needed or desired maximum voltage improvement characteristic, the fault current capability characteristic and the maximum voltage improvement characteristic are used to determine an impedance to be added to the distribution network. An electrical component (e.g., inductor) having the proper line impedance value is electrically connected within the distribution line network.
The ampacity of the distribution network (i.e., strength of the system) is determined so that the electrical component can be appropriately sized to meet the present and future ampacity of at least one load (e.g., customer facility).
The voltage recovery system is electrically connected to the distribution network between the electrical component and the utility power network to transfer real power and reactive power to the distribution network. In certain applications, power factor correction is added to the distribution line of the network. The power factor correction is in the form of at least one shunt-connected capacitive element to the distribution line. In some cases, a first capacitive element to the distribution line followed by adding, after a delay period, a second capacitive element to the distribution line. In essence, the capacitive elements are added in steps to avoid producing too large of change in voltage (increase or decrease) too quickly and to follow load changes.
Another aspect of the invention relates to a voltage recovery system for use with a utility power network that includes a transmission line network for carrying a voltage within a first predetermined voltage range and a distribution line network, electrically connected to a transmission network. The distribution network carries a voltage within a second predetermined voltage range, lower than the first predetermined voltage range, and has distribution lines coupled to at least one load. The voltage recovery system includes a voltage recovery device configured to provide real and reactive power to the distribution line network and the at least one load, and a component having an impedance value selected such that the voltage recovery device provides the real and reactive power to the distribution line at a sufficient level and for a sufficient duration to maintain the voltage at the at least one load within the second predetermined voltage range above a predetermined threshold.
Embodiments of this aspect of the invention may include the following features. The impedance value of the inductor is determined on the basis of the fault current capability characteristic and the maximum voltage improvement characteristic. The electrical component (e.g., an inductor) has an ampacity rating at least that of an ampacity rating of the distribution network (or protected load). The voltage recovery system further includes a power factor correction device (e.g., capacitive elements) connected to the distribution line. In certain applications, the power correction device includes a first capacitive element connected to the distribution line; and a second capacitive element connected to the distribution line after a delay period. The voltage recovery system can further include an inverter to control the level of real power and level of reactive power transferred between the voltage recovery system and the distribution network.
Among other advantages, a method is provided for determining whether a particular voltage protection level desired by a factory load is met by the existing distribution network. The voltage protection is generally determined by personnel at the factory load seeking to avoid production losses due to momentary voltage sags caused by faults. If the particular voltage protection desired by the factory load is not met by the existing distribution network, a voltage recovery device can be connected to the distribution line to provide additional voltage xe2x80x9cboostxe2x80x9d during the sag.
In addition, if the particular voltage recovery device is not capable of providing the level of protection desired by the factory load, additional impedance in the form of an inductor can be inserted in the distribution line to change the fault current capability of the distribution line, thereby improving the amount of voltage protection provided by the voltage recovery device. Thus, the voltage recovery system and its method of operation provides voltage protection to a factory load by supplying additional voltage to the distribution line in response to a detected sag event on the distribution line. Energy stored by the voltage recovery system bridges the voltage sag so that the factory load sees smooth, uninterrupted power.
Other features and advantages of the invention will be apparent from the following description of a presently preferred embodiment, and from the claims.